The objective of this study was to review the literature concerning laboratory tests to detect hepatitis C virus infection, which have been available since 1989. The diagnosis of hepatitis C is mainly based on serological techniques and on molecular techniques. Serological techniques to detect hepatitis C virus antibodies are the method of choice to identify past or present infection. There are two types of serological assays: highly sensitive enzyme-linked immunosorbent screening assays; and more specific immunoblot techniques, which are used as supplemental or confirmatory tests. With respect to molecular diagnostic techniques, there are several types of assays. One such assay detects viral RNA. It is useful for diagnosis in such situations as the early stages of infection, with immunosuppressed patients, and with persons who have a low probability of infection. Molecular assays are also recommended before treatment with interferon and ribavirin, in order to monitor response to treatment. Other assays allow determination of viral load by either target amplification (as in polymerase chain reaction) or signal amplification (as in branched-DNA). Determining the hepatitis C virus genotype is possible using either molecular techniques or serotyping. Determining viral load and genotype is useful for planning the duration of interferon and ribavirin treatment. There have been major advances in the diagnosis of hepatitis C in the past decade Improvements in the sensitivity and specificity of antibody tests have provided faster, less expensive diagnoses. However, more accurate assays are still needed for such groups as immunosuppressed persons and acute hepatitis patients.